I agree it should go in. I wonder about how this relates to 'comparing' which we already have in Data.Ord. Does adding 'on' mean we should remove 'comparing' ? I would suggest we keep both and add 'equating' to Data.Eq too, for symmetry with 'comparing'. Why? Because for the cases covered by comparing/equating I think the code feel simpler and reads more naturally. In my own code I think I would use comparing/equating when using the ordinary Ord/Eq instances and use `on` when I was using a non-standard comparison. Duncan On Thu, 2006-11-02 at 18:00 +0100, Nils Anders Danielsson wrote:
The function on defined by
(*) `on` f = \x y -> f x * f y
is convenient when using functions like sortBy: sortBy (compare `on` fst), for example. It also makes the code more readable.
Furthermore I consider on to be above the Fairbairn threshold, since * we get rid of two lambdas, * we get rid of the duplication of p, * on has some nice algebraic properties (documented in the patch) * and, most importantly, it is easier at a glance to understand (*) `on` p than to understand \x y -> p x * p y (assuming one knows about on).